1. Field of the Invention
The present invention relates to an apparatus for detecting the concentration of alcohol contained in liquid such as fuel supplied to a combustor or the like. More particularly, the present invention relates to an apparatus for detecting the concentration of alcohol contained in alcohol-mixed fuel used for an automotive internal combustion engine.
2. Description of the Related Art
In recent years, the U.S. and various countries in Europe have been tried to use mixed fuel in which gasoline is mixed with alcohol such as methanol as automotive fuel in order to reduce the consumption of oil and to reduce the air pollution. However, if alcohol-mixed fuel is directly used in an engine which is tuned to match an air-fuel ratio of gasoline, then difficulties with engine operation will occur due to a lean air-fuel ratio which results from the fact that the theoretical air-fuel ratio of alcohol is lower than that of gasoline. Therefore, it is required to detect the alcohol content or the concentration of alcohol contained in the alcohol-mixed fuel so as to adjust the air-fuel ratio and the ignition timing according to the detected value of the alcohol concentration.
In known techniques, the alcohol content is determined by detecting either the dielectric constant or the refractive index of alcohol-mixed fuel. One method of detecting the dielectric constant is disclosed in Japanese Patent Laid-Open No.4-262249, which will be described below, referring to the figures.
FIG. 8 is a block diagram illustrating a conventional apparatus for detecting the alcohol concentration of alcohol-mixed fuel containing alcohol such as methanol, according to the method disclosed in Japanese Patent Laid-Open No.4-262249. As shown in FIG. 8, the conventional apparatus for detecting the alcohol concentration comprises a sensor A and a detecting circuit B. The sensor A comprises an LC parallel resonant circuit which provides a resonant frequency f corresponding to the dielectric constant .epsilon. of alcohol-mixed fuel. FIG. 9 shows a simplified equivalent circuit of this LC parallel resonant circuit, which will be described in more detail later. The sensor A includes a barrel-shaped insulating tube 1 made of an insulating material such as ceramic or oil-resistant plastic. The barrel-shaped insulating tube 1 is provided with a rim 1a extending outward from an open end of the insulating tube 1. The sensor A also includes an electrically-conductive electrode 3 having a cylindrical shape coaxial to the barrel-shaped insulating tube 1 wherein the conductive electrode 3 is disposed in the barrel-shaped insulating tube 1 such that the conductive electrode 3 is substantially parallel to the barrel-shaped insulating tube 1. The sensor A also includes a single-layer winding coil 4 wound around the outside of the barrel-shaped insulating tube 1 such that the coil 4 is opposed to the conductive electrode 3 which is disposed within the barrel-shaped insulating tube 1. Both ends of the single-layer winding coil 4 are electrically connected to leads 4a and 4b, respectively, as shown in FIG. 8. As shown in FIG. 8, the lead 4a is connected to one end of a resistor Rs (10) provided in the detecting circuit B, which will be described in more detail later. The lead 4b is grounded in the detecting circuit B. A fuel path 2 is formed between the outer surface of the conductive electrode 3 and the inner surface of the single-layer winding coil 4 via the wall of the barrel-shaped insulating tube 1 so that fuel to be detected can flow through the fuel path 2. The conductive electrode 3 is also provided with a flange 5. The flange 5 is fixed via a fuel seal 8 to the rim 1a of the barrel-shaped insulating tube 1. The flange 5 may be formed as an integral part of the conductive electrode 3. A fuel chamber is formed with these elements described above. A pair of nipples 6 are provided through the flange 5 so that the fuel may be introduced into the fuel path 2.
The detecting circuit B for detecting the resonant frequency f provided by the sensor A will be described below. The detecting circuit B comprises: a resistor Rs (10) which is electrically connected to the lead 4a of the single-layer winding coil 4 in such a manner that a series circuit is formed to be composed of the resistor Rs (10) and the single-layer winding coil 4; a zero-degree phase comparator 11 for making comparison in phase between the voltage signals appearing at each end of the resistor 10; a low-pass filter 12 electrically connected to the zero-degree phase comparator 11 for smoothing the output of the zero-degree phase comparator 11 so as to provide a DC voltage corresponding to the difference in phase between the above-described voltage signals; a comparison integrator 13 which makes comparison between the DC voltages provided from the low-pass filter 12 and a predetermined reference voltage Vref corresponding to the phase of 0.degree. so as to provide an output signal representing the integration of the differences obtained by the comparison; a voltage-controlled oscillator 14 electrically connected to the comparison integrator 13 for providing an oscillating voltage having an oscillation frequency corresponding to the output of the comparison integrator 13; a frequency divider 16 electrically connected to the voltage-controlled oscillator 14 for providing a signal fout to the outside circuit wherein the signal fout is obtained by dividing the output frequency of the oscillating signal provided by the voltage-controlled oscillator 14; and an amplifier 15 electrically connected to the voltage-controlled oscillator 14 for amplifying the oscillating output provided by the voltage-controlled oscillator 14 so as to apply the amplified output to the series circuit comprising the resistor 10 and the single-layer winding coil 4.
Now, the operation of the conventional alcohol concentration detector will be described below. As described above, the sensor A, shown in FIG. 8, comprises an LC parallel resonant circuit which can be substantially represented by an equivalent circuit shown in FIG. 9, where L is the inductance of the single-layer winding coil 4, Cf is the capacitance distributed between the single-layer winding coil 4 and the conductive electrode 3 wherein the capacitance Cf varies depending on the dielectric constant .epsilon. of the fuel passing through the fuel path 2, Cs is the capacitance associated with a dielectric or an insulating material included in the barrel-shaped insulating tube 1 which protects the single-layer winding coil 4 from the fuel, and Cp is the total stray capacitance associated with the lead 4a of the single-layer winding coil 4 and the input capacitance of the zero-degree phase comparator 11 and the like wherein the capacitance Cp is independent of the dielectric constant .epsilon. of the fuel.
If the varying frequency of the output signal, which is applied by the amplifier 15 to the lead 4a of the sensor A, is varied, the sensor A exhibits an LC parallel resonance state. The parallel resonance frequency f can be substantially described by using the notations in the equivalent circuit as follows: ##EQU1## where k, a, and b are constants which are determined for example by the shape of the sensor such as the diameter and the thickness of the barrel-shaped insulating tube 1, the dielectric constant of the material of the barrel-shaped insulating tube 1, the distance between the conductive electrode 3 and the single-layer winding coil 4, the self-inductance of the single-layer winding coil 4.
As can be seen from the equation 1, the parallel resonance frequency f depends on the dielectric constant .epsilon. of the fuel, therefore the parallel resonance frequency decreases with increasing dielectric constant .epsilon. of the fuel. For alcohol-mixed fuel containing various contents of gasoline and alcohol, the output frequency of the sensor A, that is the parallel resonance frequency f, varies according to the alcohol content (%) as shown in FIG. 10. FIG. 10 shows the case of mixed fuel containing methanol. As can be seen, the signal fout corresponding to the parallel resonance frequency f is provided from the detecting circuit B, then the dielectric constant .epsilon. of the alcohol-mixed fuel and thus the alcohol content (%) can be detected.
The detecting circuit B shown in FIG. 8 is configured so that the parallel resonance frequency f can be detected. The operation of this detection circuit B will be described below. In a state in which alcohol-mixed fuel is passing through the fuel path 2, the amplifier 15 provides a high frequency voltage signal to the series circuit composed of the resistor 10 and the single-layer winding coil 4. Then, the signals appearing at each end of the resistor 10, those are the high frequency voltage signal across the above-described series circuit and the high frequency voltage signal across the single-layer winding coil 4, are applied to the zero-degree phase comparator 11, which compares these signals. If, for example, a high frequency signal having a phase which is the same as that of the output frequency f of the sensor A is provided by the amplifier 15 to the above-descried series circuit comprising the resistor 10 and the single-layer winding coil 4, then the current-voltage phase of the sensor A becomes 0.degree.. As a result, the phase difference between the high frequency voltage signals at each end of the resistor 10 becomes 0.degree.. If, a high frequency voltage signal having a frequency lower than that of the output frequency f of the sensor A is applied to the above-described series circuit, then the current-voltage phase of the sensor A becomes ahead of 0.degree., and thus, the phase difference between the high frequency voltage signals at each end of the resistor 10 becomes larger than 0.degree. as defined relative to the phase of the high frequency voltage signal applied to the above-described series circuit.
Then, the output of the zero-degree phase comparator 11 is converted by the low-pass filter 12 to a DC voltage corresponding to the phase difference. This DC voltage as well as a predetermined reference voltage Vref is input to the comparison integrator 13, which makes integration of the differences between these input signals. The output of the comparison integrator 13 is then input to the voltage-controlled oscillator 14. Thus, via the voltage-controlled oscillator 14 and the amplifier 15, the high frequency voltage signal is applied to the above-described series circuit composed of the resistor 10 and the single-layer winding coil 4. As can be seen from the above description, a phase-locked loop is formed in the detecting circuit B. In this phase-locked loop, the voltage-controlled oscillator 14 is controlled so that the phase difference between the signals appearing at each end of the above-described resistor 10 becomes 0.degree., that is, so that the phase difference between the voltage signal applied to the above-described series circuit and the voltage signal applied to the single-layer winding coil 4 becomes 0.degree.. As a result, the voltage-controlled oscillator 14 always oscillates at a frequency which corresponds to the parallel resonance frequency f of the sensor A. The oscillation frequency of the voltage-controlled oscillator 14 which is in the high frequency range is divided by the frequency divider 16 down to a frequency output fout having a frequency low enough to measure the output. In this way, the dielectric constant .epsilon. can be determined from the frequency output fout according to equation 1, and thus the alcohol content (%) can be detected.
In the conventional apparatus fore detecting the alcohol concentration, the dielectric constant .epsilon. of alcohol-mixed fuel is determined from the output frequency fout of the frequency divider 16 in the detecting circuit B, based on the assumption that the dielectric constant of the alcohol-mixed fuel and the dielectric constant .epsilon. of the barrel-shaped insulating tube 1 do not vary. However, the actual dielectric constant .epsilon. of the alcohol-mixed fuel and also the actual dielectric constant of the barrel-shaped insulating tube 1 vary with change of temperature. As a result, the output frequency fout shows a significantly large change even for the constant alcohol content. Therefore, accurate detection of the alcohol content is impossible if the dielectric constant .epsilon. is determined simply by the output frequency fout.